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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A simple question, I hope, on almanac SHA
From: Stan K
Date: 2013 Apr 22, 09:11 -0400
From: Stan K
Date: 2013 Apr 22, 09:11 -0400
Herbert,
Thanks for going through this exercise. It appears to confirm what seemed intuitive to me - use the middle of the three-day period, not a time only one-third of the way in.
Nevertheless, I have already sent the question to Catherine Hohenkerk, who has been very helpful in the past.
Stan
-----Original Message-----
From: Herbert Prinz <herbert@heavenlysoftware.org>
To: slk1000 <slk1000@aol.com>
Sent: Sun, Apr 21, 2013 4:08 am
Subject: [NavList] Re: A simple question, I hope, on almanac SHA
From: Herbert Prinz <herbert@heavenlysoftware.org>
To: slk1000 <slk1000@aol.com>
Sent: Sun, Apr 21, 2013 4:08 am
Subject: [NavList] Re: A simple question, I hope, on almanac SHA
After a few spot checks on two stars I convinced myself that the chosen epoch for the tabulated SHA is noon of the middle day. Theoretically, the value given could also be the average found by integration over the entire 3 day interval. The latter would be the perfectly correct method as it minimizes the average look up error, but I doubt that it has been used. It would be overkill. There might not be a single day where it makes a difference for any star. Consequently, statistics cannot decide whether the method was applied.
My spot checks refute the hypothesis that the epoch is 00:00 of the second day. In other words, I found counterexamples.
The analysis is based on a critical table of SHA values generated with MICA 2.0. The current version is apparently 2.2.2. I could not find out from the USNO website what the updates are and whether they are crucial to the validity of my test.
Does anyone know what the updates are? Do they affect the ephemeris data?
Herbert Prinz
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